Image reading devices used in facsimile machines, copiers, or multifunction image forming apparatuses typically include a document illuminating device to illuminate originals. In document illuminating devices, light-emitting diodes (LEDs), which are point sources, are widely used as light sources in response to the recent need for speeding up the rise time, extending the useful life of the light source, and saving energy.
To function as point sources, LEDs must have a small light-emitting surface. However, lighting systems constructed of only such LEDs having a small light-emitting surface cannot provide a sufficient amount of light, and further the illuminance distribution in a sub-scanning direction is not uniform.
To overcome the above-described difficulties, for example, light guides may be used to guide the light emitted from the LEDs of the document illuminating device. It is necessary to optimize the light sources such as LEDs and the light guide to condense light efficiently and attain uniform illuminance distribution in high-quality image reading devices such as general-purpose scanners or those used in digital or analogue image forming apparatuses such as copiers and multifunction machines capable of either monochrome or multicolor image formation.
For example, JP-2008-035036-A proposes a document illuminating device including multiple LEDs arranged in a main scanning direction, supported on an LED board, and a light guide for guiding the light emitted from the LEDs to an irradiation range of the original. The light guide is also supported by the LED board, and is long in the main scanning direction in which the LEDs are arranged. The light guide includes multiple engagement projections spaced at an interval in the main scanning direction to attach the light guide to the LED board.
The light guide proposed in JP-2008-035036-A, however, has a drawback in that, depending on the presence of the engagement projection of the light guide, it is possible that the intensity of light emitted from the light guide fluctuates in the main scanning direction. More specifically, in the area where the engagement projection is formed, the light emitted from the LED partly passes through the engagement projection of the light guide and is deflected from the irradiation area of the original. By contrast, in the area where the engagement projection is not formed, all the light emitted from the LED can be internally reflected totally (total internal reflection) and directed to the irradiation area of the original.